Doing business in agriculture or fisheries in the Mekong Delta is not looking like such a good long-term bet these days. For now, the Mekong's waters, rich in nutrients, rise and fall as they have for thousands of years, and the yield is good. Those days, however, are coming to an end.

Laos is building the Xayaburi hydroelectric dam to block the Mekong, despite promising its neighbours that share the river it would not. The dam will irreversibly change the nature of the river already under stress from several dams upstream in Yunnan. Cambodia and Vietnam, which lie downstream, pleaded for delay, to no avail, at a recent meeting of the Mekong River Commission, a think tank in all but name.

After the communist victories of 1975, comrades on both sides had at first ensured that Laos toed the line set by Hanoi. But that line has withered for two reasons. One, platitudes and aid have poured in from Beijing over the past decade, accompanied by investment from public and private Chinese firms. Two, the legacy of wartime suffering that maintained Vietnamese influence has faded along with the departure of old soldiers.

Their place in Vientiane has been taken by hard-headed leaders who see the personal and national interest in turning land into capital.

Since the 1960s, some experts have argued the best bet for Laos is to dam the Mekong and export electricity to China, Thailand and Vietnam. Vietnamese firms, too, have built dams on Mekong tributaries in Laos and Vietnam.

Therein lies the rub. Hanoi wants the natural and electrical power of the Mekong. But one precludes the other. A river without dams delivers high environmental quality, providing high yields of rice, fruit and fish from the Mekong Delta, guaranteeing national food security and billions in export dollars. A river with dams also promises to deliver benefits, but may not do so because of climate change and corruption.

Vietnam, maximising gas, carbon capture, energy efficiency, and wind and solar, might do without hydroelectricity. There isn't, however, any substitute for the security of strong ecosystems and farmers and fishers who know what they are doing. The key to that security is a Mekong that works according to the design of nature.

If the benefits of food and national security are valued, then they are going to have to be paid for. If the beneficiaries won't pay for Laotians to behave in one way, then those who covet electricity will pay for them to behave in another that could lead to more dams like the Xayaburi.

It may not be pretty but then realpolitik never is. Governments and investors banking on abundant food from Indochina are on notice to make plans for a Mekong future very different from the past.

David Fullbrook is a sustainability economist

This article appeared in the South China Morning Post print edition as Mekong's natural riches are worth paying for